A force that causes a displacement of an object does work. Work:Equal to the magnitude of the force, F, times the magnitude of the displacement. Measured in Joules (J).
The Joule is equal to:
or:
Work is done only when components of a force are parallel to a displacement. If your force is other than horizontal, only the horizontal component of your applied force causes displacement and does work.
The sign of work is important Work is positive when the component of force is in the same direction as the displacement. Work is negative when the force is in the direction opposite the displacement.
If the net work is positive, the object speeds up and the net force does work on the object. If the net work is negative, the object slows down and work is done by the object on another object.
Practice:
1. How much work is done on a vacuum cleaner pulled 3.0 m by a force of 50.0 N at an angle of 30.0 degrees above the horizontal?
(Answer: 130 J)
If many constant forces are acting on an object, the equation remains the same but you use the net forces and it results in the net work.
Section 5-2: Energy
Kinetic energy depends on speed and mass. Kinetic energy: A scalar quantity that is also measured in Joules.
Kinetic energy theorem: Potential energy is stored energy. Potential energy: Present in an object that has the potential to move because of its position relative to some other location. It depends not only on the properties of an object but also on the object's interaction with its environment. Gravitational potential energy depends on height from a zero level. Gravitational potential energy:The energy associated with an object due to the object's position relative to a gravitational source. The SI unit is the Joule.m is the mass of the object, g is acceleration due to gravity, and h is the height of the object.
Elastic potential energy depends on distance compressed or stretched. The length of a spring when no external forces are acting on it is called the relaxed length of the spring. When the spring is compressed or stretched, elastic potential energy is stored. The amount of energy depends on the distance the spring is compressed or stretched from its relaxed length. Elastic potential energy: Potential energy present in a spring.
k= spring constant (A flexible spring has a small spring constant, where a stiff spring has a large spring constant.)
Practice:
1. A 7.00 kg bowling ball moves at 3.00 m/s. How much kinetic energy does the bowling ball have? How fast must a 2.45 g table-tennis ball move in order to have the same kinetic energy as the bowling ball? Is this speed reasonable for a table-tennis ball?
Answer:
This speed is much too high to be reasonable for a table-tennis ball
2. On a frozen pond, a person kicks a 10.0 kg sled, giving it an initial speed of 2.2 m/s. How far does the sled move if the coefficient of kinetic friction between the sled and the ice is .10?
3. A 70.0 kg stuntman is attached to a bungee cord with an unstretched length of 15.0 m. He jumps off a bridge spanning a river from a height of 50.0 m. When he finally stops, the cord has a stretched length of 44.0 m. Treat the stuntman as a point mass, and disregard the weight for the bungee cord. Assuming the spring constant of the bungee cord is 71.8 N/m, what is the total potential energy relative to the water when the man stops falling?
Answer:
Section 5-3: Conservation of energy
Mechanical energy: The sum of kinetic energy and all forms of potential energy associated with an object or group of objects.
Mechanical Energy is often conserved. Conservation of mechanical energy:
or:
Energy conservation occurs when acceleration varies.
On a frictionless plain, mechanical energy is conserved. Mechanical energy is not conserved in the presence of friction.
Practice:
1. Starting from rest, a child zooms down a frictionless slide from an initial height of 3.00 m. What is her speed at the bottom of the slide? Assume she has a mass of 25.0 kg.
Answer:
Section 5-4: Power
Power:The rate at which work is done. Measured in watts (W). Or horsepower.
Machines with different power ratings do the same work in different time intervals.
Chapter 5
Section 5-1: Work
A force that causes a displacement of an object does work.Work: Equal to the magnitude of the force, F, times the magnitude of the displacement. Measured in Joules (J).
The Joule is equal to:
or:
Work is done only when components of a force are parallel to a displacement.
If your force is other than horizontal, only the horizontal component of your applied force causes displacement and does work.
The sign of work is important
Work is positive when the component of force is in the same direction as the displacement. Work is negative when the force is in the direction opposite the displacement.
If the net work is positive, the object speeds up and the net force does work on the object. If the net work is negative, the object slows down and work is done by the object on another object.
Practice:
1. How much work is done on a vacuum cleaner pulled 3.0 m by a force of 50.0 N at an angle of 30.0 degrees above the horizontal?
(Answer: 130 J)
If many constant forces are acting on an object, the equation remains the same but you use the net forces and it results in the net work.
Section 5-2: Energy
Kinetic energy depends on speed and mass.
Kinetic energy: A scalar quantity that is also measured in Joules.
Kinetic energy theorem:
Potential energy is stored energy.
Potential energy: Present in an object that has the potential to move because of its position relative to some other location. It depends not only on the properties of an object but also on the object's interaction with its environment.
Gravitational potential energy depends on height from a zero level.
Gravitational potential energy: The energy associated with an object due to the object's position relative to a gravitational source. The SI unit is the Joule. m is the mass of the object, g is acceleration due to gravity, and h is the height of the object.
Elastic potential energy depends on distance compressed or stretched.
The length of a spring when no external forces are acting on it is called the relaxed length of the spring. When the spring is compressed or stretched, elastic potential energy is stored. The amount of energy depends on the distance the spring is compressed or stretched from its relaxed length.
Elastic potential energy: Potential energy present in a spring.
k= spring constant (A flexible spring has a small spring constant, where a stiff spring has a large spring constant.)
Practice:
1. A 7.00 kg bowling ball moves at 3.00 m/s. How much kinetic energy does the bowling ball have? How fast must a 2.45 g table-tennis ball move in order to have the same kinetic energy as the bowling ball? Is this speed reasonable for a table-tennis ball?
Answer:
This speed is much too high to be reasonable for a table-tennis ball
2. On a frozen pond, a person kicks a 10.0 kg sled, giving it an initial speed of 2.2 m/s. How far does the sled move if the coefficient of kinetic friction between the sled and the ice is .10?
3. A 70.0 kg stuntman is attached to a bungee cord with an unstretched length of 15.0 m. He jumps off a bridge spanning a river from a height of 50.0 m. When he finally stops, the cord has a stretched length of 44.0 m. Treat the stuntman as a point mass, and disregard the weight for the bungee cord. Assuming the spring constant of the bungee cord is 71.8 N/m, what is the total potential energy relative to the water when the man stops falling?
Answer:
Section 5-3: Conservation of energy
Mechanical energy: The sum of kinetic energy and all forms of potential energy associated with an object or group of objects.Mechanical Energy is often conserved.
Conservation of mechanical energy:
or:
Energy conservation occurs when acceleration varies.
On a frictionless plain, mechanical energy is conserved.
Mechanical energy is not conserved in the presence of friction.
Practice:
1. Starting from rest, a child zooms down a frictionless slide from an initial height of 3.00 m. What is her speed at the bottom of the slide? Assume she has a mass of 25.0 kg.
Answer:
Section 5-4: Power
Power: The rate at which work is done. Measured in watts (W). Or horsepower.Machines with different power ratings do the same work in different time intervals.